I'm new to docker.

I'm trying to run the nginx/alpine container and getting this error:

2025/08/10 01:09:55 [emerg] 1#1: unknown "flask_port" variable nginx: [emerg] unknown "flask_port" variable

Strangely, there is no flask_port reference in my codebase (having grepped for it in ./*). There is FLASK_PORT, but it seems unlikely that that the case for the missing variable would get changed before outputting to the log file, right?!

I have rebuilt the container many times and cleared the cache. But the message persists.

There is a reference to FLASK_PORT in the .conf file but changing it doesn't change the error message, so I assume it isn't coming from there.

No reference to it in docker-compose.yml or Dockerfile.

Any tips on how to troubleshoot something like this? 🙏